The present invention relates to a semiconductor device having MOS-type field-effect transistors, and particularly to a semiconductor device which enables elements to be formed in small sizes and with a high degree of integration to be realized, and to a process for manufacturing the same.
The trend toward producing semiconductor devices in highly integrated form in recent years has required scaling-down MOS (metal oxide semiconductor)-type field-effect transistors (MOSFET's). However, scaling-down has resulted in the development of a so-called short-channel effect, imposing problems such as reduced threshold voltage Vth and the like. Therefore, it has been attempted to extinguish the short-channel effect by providing the channel region of MOS-type field-effect transistors with a semiconductor region having the same conductivity type as that of the substrate.
For instance, IEEE Transactions on Electronic Devices (Vol. ED-29, No. 4, 1982, pp. 607-610) discloses a device that is illustrated in FIG. 8. That is, in an MOS-type field-effect transistor 1, a p-type impurity layer 4 of a high concentration is formed on the lower side of not only a gate electrode but also n.sup.+ -type source and drain regions 3. Under the gate electrode, furthermore, a region 2 implanted with phosphorus ions is formed, thereby to form a buried channel which increases the threshold voltage Vth to reduce the short-channel effect.
According to this construction, however, it has been found by the inventors of the present invention that since the contact area increases between the p-type region 4 and the n.sup.+ -type source and drain regions 3, the junction capacity increases between them to make it difficult to operate the elements at high speeds.
The above-mentioned literature further proposes a device in which a p-type region is formed under the channel only. Even with this construction, however, the p-type region and the source and drain regions are contacted with each other in relatively wide areas, making it difficult to fulfill the object of high-speed operation.